The invention relates to a circuit arrangement for transforming present 2.sup.n global addresses used in a control engineering system having several users into 2.sup.m local addresses used in one of the local units of the system, comprising at least one memory.
In distributed control engineering systems having a plurality of units operating in accordance with the principle of source addressing, an individual unit, as a rule, is only utilizes a subset of the messages transmitted via the system. From the point of view of the individual unit, therefore, the source address space which comprises all addresses which are global for the control engineering system, are only very loosely occupied. To achieve an economic utilization of the communication memory required for communicating with the other units of the control engineering system in the individual user, a smaller space than the source address space is used for internal referencing and storing of the communication data. The consequence of this is that the source address space must be transformed into the internal address space of the individual unit. The transformation is determined by a separate transformation function of each individual unit. This function distinctly allocates to each global address of the source address space used by each unit a separate local internal address and to all other global addresses of the source address space not used by the unit a common local internal address. The transformation function is therefore a discrete function.
Prior systems calculate such discrete functions by means of a complete function table which is stored in a memory. This method is very fast because when the argument of the function, that is to say the global address, is applied to the address inputs of the memory, the desired local address immediately appears as associated function value at its data outputs. However, for a complete tabulation of the transformation function, a storage space of N Log.sub.2 M bits is required where N designates the size of the source address space, M the size of the internal address space and the special brackets are intended to indicate that the matter bracketed by them is to be rounded up to the next higher integral number. In a system typical of present-day applications and having N=2.sup.24 and M=2.sup.8, a storage requirement of 16,777,216.times.8 bits would be the result. To implement such a memory, for example with memory chips of 8K.times.8 bits, which are inexpensive at present, more than 2000 memory chips would be required which means an outlay which is no longer practicable.
It is also known to store only the global addresses used by the individual unit, together with the local addresses associated with them, in a list stored in a memory. This solution requires the lowest memory capacity. In this solution, however, the list must be searched in each case by the user for transformation. In the individual case, this can take considerable time which limits the data rate which can be transmitted in the control engineering system in a manner which can no longer be tolerated for present-day requirements.